The present invention relates to the carrier and code tracking in the presence of signal fading. Specifically, an adaptive smoother is used to compensate for signal fading arising for example due to ionospheric scintillation in the propagation of GPS signals.
The Global Positioning System (GPS) relies upon GPS signals transmitted through the ionosphere between orbiting GPS satellite transmitters and ground based receivers that acquire and lock onto the GPS signals. As the GPS signals traverse the ionosphere, scattering of energy takes place due to irregularities in the ionosphere, primarily at altitudes at 300-400 Km. Small-scale variations in ionospheric electron density result in rapid variations in the amplitude and phase of the received GPS signal, known as scintillation. Scintillation effects get more severe during times of peak solar activity. Possible effects of this scintillation phenomenon on the overall performance of GPS navigation will appear as degraded measurement accuracy obtained by the GPS receiver and by the reference receivers in case of wide area augmentation systems, (WAAS). In terms of scintillation activity, the regions of greatest concern for most GPS users include polar latitudes, auroral latitudes during severe magnetic storms, and equatorial regions during years of high solar flux and extending approximately 30xc2x0 either side of the geomagnetic equator. The regions of severest ionospheric activity are concentrated within 10xc2x0 wide bands centered about 15xc2x0 either side of the geomagnetic equator. In these regions, GPS L-band signals from beacon satellites have been observed to fade by up to 30 dB. These periods of fade can last for several hours with period of no fading in between. During intense ionospheric conditions, the GPS signals suffer simultaneously from amplitude fading and rapid phase changes causing performance degradation of the tracking loop of the GPS receiver.
Ionospheric physical modeling has included diffraction models from isolated irregularities, perturbation models of approximation, and the phase screen models. These models have been used for the purpose of evaluating the impact of amplitude and phase scintillation on the GPS receiver. The results show that under Rayleigh fading conditions representing strong scintillation, there is a performance degradation of up to about 6-7 dB due to amplitude scintillation. Tracking Phase lock loops (PLL) have numerically controlled oscillators (NCO) for phase tracking but do not follow amplitude variations. Current tracking loop designs do not solve the problem of amplitude scintillation. Kalman filter based phase lock loops have been used for phase estimation in combination with fixed delay phase smoothers. In a fixed delay phase smoother, the amplitude A(k) is either assumed known for all time k or is assumed equal to a known constant. Alternatively, a Kalman filter based phase lock loop is used in combination with a first order amplitude estimator for unknown amplitudes for improved phase estimation. Existing low pass filters have been designed to sufficiently estimate the average signal power to determine the signal to noise power ratios (SNR) but are inadequate for following the instantaneous amplitude variations caused by scintillation. In GPS receivers, the SNR is not determinative, but rather, the accuracy of the phase estimation is important.
In the presence of phase scintillation, the GPS receiver will track the composite dynamic phase process comprising relative dynamics of the GPS satellite and receiver, any receiver reference oscillator noise and the scintillation phase dynamics. The increased phase dynamics due to the ionospheric scintillation will result in increased tracking errors, cycle slips or possibly loss of lock. For this case, a multistage estimator structure disclosed in U.S. Pat. No. 5,019,824, by Dr. R. Kumar, entitled xe2x80x9cMultistage Estimation of Received Carrier Signal Under Very High Dynamic Conditions of the Receiver,xe2x80x9d issued on May 28, 1991 can be applied. This architecture comprises more than one estimation stage wherein the succeeding estimation stages process the error signals generated by the preceding stages to provide an overall estimate that is better than can be obtained by a traditional single stage estimator under certain conditions. A two-stage specialization of the multistage estimator of the Kumar patent has been proposed to solve the problem of estimation in the presence of phase scintillation. Simulation examples show that a second estimation stage results in improvement of the tracking error by about 6.5 dB over the single stage estimator. While the multistage estimator can be applied to the problem of phase scintillation, it does not provide a solution to the degradation caused by the amplitude scintillation.
The scintillation effects appear as degraded tracking accuracy and may also cause receiver loss of lock and longer acquisition times. Simulation results show that the slower the amplitude fading is compared to the tracking loop bandwidth, the more is the performance degradation. Because the fade rate is expected to be relatively slow at the operating GPS frequencies, the simulation results show the significance of ionospheric scintillation on the GPS signals especially under solar max conditions and in the equatorial and polar regions wherein the GPS signals may experience deep fades. Such fades may result in disruption of GPS service especially to some safety critical applications such as GPS based aviation including aircraft precision approach and landing. Amplitude scintillation degrades the tracking performance through large coherent phase errors and code tracking errors in communication and navigation receivers. Ionospheric scintillation causes amplitude variations of received signals that result(s) in phase estimator errors or code tracking errors. These and other disadvantages are solved or reduced using the invention.
An object of the present invention is to improve receiver performance in the presence of amplitude scintillation of a transmitted signal.
Another object of the invention is to provide real-time estimation of the scintillation amplitude.
Yet another object of the invention is to provide an adaptive smoother for improved carrier phase estimations in the presence of varying scintillation amplitudes
Still another object of the invention is to provide an adaptive smoother having a Kalman filter based phase lock loop for phase estimations, in combination with a high order amplitude estimator for providing rapidly varying dynamic amplitude variations, and in combination with a fixed delay phase smoother for improved phase estimation.
The present invention is an adaptive smoother for solving the problem of coherent receiver performance degradation in the presence of amplitude variations, such as those caused by ionospheric amplitude scintillation typically experienced by the communication or navigation signals, such as GPS signals, resulting in deep signal fades. The adaptive smoother provides real-time estimations of the scintillation amplitude and the resulting phase estimations with reduced phase estimation errors. The adaptive smoother is capable of tracking amplitude variations under realistic channel fade rates. As the impact of amplitude fluctuations is most dominant under relatively slow, but deep fades, the adaptive smoother provides improved estimates of the amplitude scintillation process and the received signal phase. The adaptive smoother is integrated into a receiver tracking loop. The tracking loop is made adaptive by including the effects of amplitude variations estimated from a high order scintillation amplitude estimator. When the tracking loop is made adaptive, the tracking performance is improved by 1-1.5 dB. The more significant improvement is achieved by making the fixed delay smoother adaptive by including the effects of amplitude variations estimated from the high order scintillation amplitude estimator. The scintillation amplitude estimator is used for providing a time varying amplitude estimation for adaptive operation for both the tracking loop and the fixed delay smoother for optimum phase smoothing for improved receiver performance by offsetting the impact of amplitude scintillation. Simulations show that the performance improvement with adaptive smoother comprising the phase lock loop, the fixed delay smoother and the scintillation amplitude estimator results in an improvement of 6-8 dB. The overall performance of adaptive smoother in the presence of amplitude fading is significantly improved as compared to an optimum Kalman filter. In the simulation examples, the adaptive smoother compensates for any loss in tracking performance due to amplitude fading.
The adaptive smoother includes the tracking loop, the scintillation amplitude estimator, and the fixed delay smoother. The adaptive smoother uses the Kalman filter based phase lock loop for phase estimations, in combination with the high order scintillation amplitude estimator for varying amplitude estimation in the presence of rapidly varying dynamic amplitude variations, and in combination with the fixed delay phase smoother for improved phase estimation. The scintillation amplitude estimator consists of a noisy amplitude estimator based on a single sample, followed by an amplitude tracking filter. The amplitude tracking filter uses a phase lock tracking loop (PLL) structure including filtering and integration functions without a numerically controlled oscillator (NCO). The scintillation amplitude estimator is capable of tracking the amplitude variations under realistic channel fade rates. As the impact of amplitude fluctuations is most dominant under relatively slow but deep fades, the amplitude estimator can provide an accurate and instantaneous estimate of the amplitude scintillation process. When the tracking loop is made adaptive with respect to the amplitude variations, simulations show that the tracking performance is improved by 1-1.5 dB. While this is a significant improvement, additional improvement is possible when the fixed delayed phase smoother is also made adaptive by processing the effects of amplitude variations. Simulations show that the performance improvement with an adaptive fixed delay phase smoother generating gain vectors with amplitude adaptation results in an improvement of 6-8 dB in tracking errors. Thus, both tracking loop and phase smoother amplitude variation adaptations are important aspects in improving the receiver performance and offsetting any impact of amplitude scintillation.
The GPS signal is code and data demodulated by conventional demodulators to provide the adaptive smoother with a carrier input signal having variable amplitude and phase. The adaptive smoother provides the final smoothed phase output to the code and data demodulator and to a navigation processor, in the case of GPS receiver computing final navigation position solutions. The fixed delay smoother introduces a delay between the smoothed phase output and the input signal during the fixed delay smoother operation. Because the fixed delay smoother delay is a small fraction of a second, the fixed delay smoother delay can be offset in most GPS applications by predicting the position estimate over the fixed delay smoother delay interval when computing the final navigation position solution. In most applications, any degradation due to such position prediction will be negligible.
The adaptive smoother applies high order amplitude estimations for dynamic amplitude variations in combination with phase estimations and smoothing for providing an improved smoothed phase output. Dynamic amplitude estimations and phase smoothing provide improved accuracy in phase estimation in the presence of high variation of scintillation amplitude variations to ameliorate the effects of amplitude fading due to ionospheric scintillation. The adaptive smoother provides a solution to the problem of high phase estimation errors resulting from scintillation amplitude variations. The improvement is stated in terms of phase accuracy, not SNR, and the 6-8 dB improvement is in terms of root mean square (RMS) phase errors. The SNR and phase errors are used to estimate the impact of scintillation amplitude variations and to verify the improvement of the adaptive smoother. Such improvement applies to a wide range of SNR conditions. The system provides 5-7 dB improvement both in terms of carrier phase and code delay tracking error variances. In GPS applications tracking error in code delay and/or carrier phase directly translates to range tracking error, to provide up to 5-7 dB improvement in terms of navigation error. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.